1. Field of the Invention
The present invention relates generally to implantable cardiac devices and, more particularly, to systems and methods for assessing autonomic tone and a patient's risk of sudden cardiac death (SCD).
2. Background Art
A ventricular premature beat (VPB) triggers fluctuation in cardiac cycle duration and a brief disturbance to arterial blood pressure, referred to as heart rate turbulence (HRT) by Schmidt et al., “Heart-rate turbulence after ventricular premature beats as a predictor of mortality after acute myocardial infarction,” Lancet 353:1390-96 (1999). Schmidt et al. define HRT as a characteristic initial acceleration and subsequent deceleration of sinus rhythm after a single VPB. The study by Schmidt et al. shows that the degree of HRT following a VPB can predict a patient's risk of SCD; for example, HRT is absent in the sinus rhythm of a high-risk patient but is present in the sinus rhythm of a low-risk patient. Schmidt et al. also defined two parameters to quantify the degree of HRT following a VPB: HRT onset, which is the initial acceleration of sinus rhythm after a single VPB, and HRT slope, which is the speed of the subsequent deceleration of sinus rhythm after a single VPB.
The degree of HRT following a VPB can also detect autonomic abnormalities. For example, U.S. Patent Application Publication No. 2003/0191403 A1, entitled “Method and apparatus for predicting recurring ventricular arrhythmias,” to Zhou et al., explains that changes in the autonomic nervous system are known contributing factors to arrhythmia development. Zhou et al. further explain that heart rate is regulated by the sympathetic and parasympathetic components of the autonomic nervous system, and that increased sympathetic activity (i.e., sympathetic tone) causes the heart rate to increase, while increased parasympathetic activity (i.e., vagal tone) causes the heart rate to decrease. Accordingly, Zhou et al. propose that monitoring changes in autonomic tone might be useful for predicting arrhythmia development.
A study by Lin et al., “Tight mechanism correlation between heart rate turbulence and baroreflex sensitivity: sequential autonomic blockade analysis,” Journal of Cardiovascular Electrophysiology, 13:427-431 (May 2002), demonstrated that because HRT is abolished when the vagus nerve is blocked, maintenance of normal HRT following a VPB is dependent on vagal tone. Lin et al. also showed that the parameters HRT onset and HRT slope are vagally dependent and, accordingly, can be used as indirect measures of vagal tone.
Additionally, Lin et al. showed that the parameters HRT onset and HRT slope are highly correlated with spontaneous baroreflex, which is described by Lin et al. as the negative feedback system that modulates dynamic fluctuations of heart rate and arterial blood pressure. A study by Mrowka et al., “Blunted arterial baroreflex causes ‘pathological’ heart rate turbulence,” Am J Physiol Regulatory Integrative Comp Physiol, 279:R1171-75 (2000), explained that a VPB followed by a compensatory pause leads to a drop in arterial blood pressure; therefore, baroreflex action is essential for compensating blood pressure. Wichterle et al., “Mechanisms involved in heart rate turbulence,” Cardiac Electrophysiology Review, 6:262-266 (2002) propose that the compensatory pause following a VPB triggers HRT as a response to the sudden decrease in arterial blood pressure.
One approach for assessing autonomic tone and a patient's risk of SCD is to measure the degree of HRT after a naturally occurring (i.e., intrinsic) premature ventricular contraction (PVC) caused by a VPB. A disadvantage of this technique, however, is it cannot be executed on-demand or at regular intervals. Another approach for assessing autonomic tone and a patient's risk of SCD, when naturally occurring PVCs are absent, is to measure the degree of HRT following an artificially induced PVC. A disadvantage of this technique, however, is that artificially inducing PVCs in the absence of naturally occurring PVCs can cause arrhythmia.
What is needed, therefore, are a system and method for detecting abnormalities in autonomic tone and assessing a patient's risk of SCD by measuring a degree of HRT absent naturally occurring or artificially induced PVCs, i.e., on demand and without posing an arrhythmic risk.